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Old 18-06-2013, 16:31   #16
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Re: 2 ohm resister

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Originally Posted by Tikka View Post
You have read a great and correct explanation.
The most important aspect was forgotten.
We talking about the boat emergency radio battery, don't we.
The whole purpose is oscillate, the house power and emergency
voltage source. System must guarantee, if anything happens
with your main, your emergency is up and fully charge.
Not killed ( discharged ) by the house.
Your resistor is back yard solution, someone who has no idea,
Simply wrong.
In this case much better solution would be to use diude instead of resister to protect the buttery.
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Old 18-06-2013, 17:01   #17
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Re: 2 ohm resister

I'm a good electrician but my weakness is things like resisters. My everyday work doesn't involve resisters. The explanations for the 2ohm resister are great.
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Old 18-06-2013, 17:11   #18
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Re: 2 ohm resister

OK Tristan! Luv your southern accent in writing! +1

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Old 18-06-2013, 18:25   #19
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Re: 2 ohm resister

You have two batteries wired in parallel, and one of the batteries has a mystery component with a .7v voltage drop wired in series, correct?

It's a diode, and probably a silicon diode. A Schottky would have a smaller voltage drop. Ordinarily you would use one diode per battery for isolation. Maybe someone didn't finish the job?

It cannot be a resistor in that configuration, because a resistor drops current, not voltage. A resistor can be used as part of a voltage divider network, but that would require more components than are described.
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Old 18-06-2013, 19:16   #20
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Re: 2 ohm resister

I'm with Andina Marie; the 2 ohm resistor + smaller battery would provide a measure of isolation from voltage fluctuations on the house bank. It's a little bit of a kludge, but it would work.

A single diode wouldn't be that great a solution because the secondary (small battery) would never charge fully, but the isolation from the house bank would be even better... the house bank could die completely without affecting the secondary battery. A Shottky diode would be better because of lower forward drop, as mentioned.

The Cadillac solution, IMHO, would be the diode plus an echo charger or similar that would ensure the secondary battery charges fully when the house bank is being charged.

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YIt cannot be a resistor in that configuration, because a resistor drops current, not voltage. A resistor can be used as part of a voltage divider network, but that would require more components than are described.
Um, a resistor limits current, and drops voltage when current is flowing through it.

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Old 19-06-2013, 03:45   #21
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Re: 2 ohm resister

maybe limiting current and voltage drop is what the person is trying to do. the smaller batter is about a 100 amp hour but the wire feeding it is like a #14.They might be trying to set it up as a trickle charger,does that make any sense.
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Old 19-06-2013, 04:44   #22
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Re: 2 ohm resister

There are several possible uses for the second small battery.

It can be be set up to act like a big capacitor and stabilise the instrument voltage during very large loads like turning on the inverter.

Another completely different use is set up a small emergency battery mounted high up. So the radio and perhaps GPS can still be operated it the bilge floods above the level of the house batteries. In some countries I believe this is even required on some vessels.

Often a small gel battery is used. These have a lower bulk charging voltage. Often a silicon diode is used and the voltage drop can be beneficial reducing he house bank bulk voltage of perhaps 14.7v down to an appropriate 14.1v. Substituting a Schottky diode, as has been suggested, is often a very bad idea.
Some of the more modern small sealed batteries are AGM. These are more tolerant of a high charge voltage so a Schottky diode may be appropriate. These small AGM batteries will sometimes accept a high charge current that will damage them. In these cases a small value resistor will limit the maximum charge current to a safe level while not not effecting the finial charge voltage.

With the simple components of a diode/ Schottky diode and or resistor a suitable crude charging profile can be created for the small batteries.
The components used will depend on the chemistry of the small battery and the charging voltages used on the house bank.
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Old 19-06-2013, 10:53   #23
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Re: 2 ohm resister

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Originally Posted by Lake-Effect View Post
I'm with Andina Marie; the 2 ohm resistor + smaller battery would provide a measure of isolation from voltage fluctuations on the house bank. It's a little bit of a kludge, but it would work.

A single diode wouldn't be that great a solution because the secondary (small battery) would never charge fully, but the isolation from the house bank would be even better... the house bank could die completely without affecting the secondary battery. A Shottky diode would be better because of lower forward drop, as mentioned.

The Cadillac solution, IMHO, would be the diode plus an echo charger or similar that would ensure the secondary battery charges fully when the house bank is being charged.



Um, a resistor limits current, and drops voltage when current is flowing through it.


I have to disagree on at least three points: The effect of a resistor in a circuit can be to drop voltage, but it does so indirectly, via E = IR. Voltage is assumed to be taken at no load unless described otherwise. If our above voltages were taken under an unspecified load, then who knows what we are looking at?

Lake-Effect possibly describes an R-2R voltage divider, with the internal resistance of the battery balanced against the resistance of the boat's electrical system. Sure, it will work after a fashion. But then what happens when you turn on an anchor light? Whatever was being powered adequately before, like say a radio, now drops out because E will go low to balance the lowered 2R.

No, a series resistor will not protect against voltage fluctuations. It will only limit the current behind them. The effect my well be a change in voltage, but that will depend on the resistance of the components in the rest of the system. E = IR again.

And yes, a sanely-designed, multiple battery multiple-diode protected system WILL charge fully. But voltage out will be lower by one diode drop. Usually not a big deal.
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Old 19-06-2013, 16:01   #24
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Re: 2 ohm resister

I was looking on line at the xantrex echo-chargers. It says that it limits the amps to 15, so with the #12 wire going from the main battery to the 100ah battery wouldn't this echo-charger work. Is that what this device does ,limits the amps to no more the 15.
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Old 19-06-2013, 18:26   #25
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Re: 2 ohm resister

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No, a series resistor will not protect against voltage fluctuations.
In the described system (big battery, 2 ohm resistor, small battery) the loads on the small battery will not see significant voltage fluctuations when the large battery has short-term high-current loads (eg windlass, starting, etc)

Regardless of the magnitude of a momentary load on the big battery, the highest possible current that can be drawn from the small battery through the resistor will be 6A... and that's only if the big battery's voltage drops to zero.

If the big battery experiences a load that drops its voltage by 1 volt, the additional load on the small battery is 1/2 = 0.5 A. Not enough to drop the small battery's voltage significantly. That's pretty good protection for the loads on the small battery.
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Old 20-06-2013, 03:01   #26
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Re: 2 Ohm Resister

OK, I stand corrected , it was resistor
IMO, it is a crazy arrangement.
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Old 20-06-2013, 14:39   #27
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Re: 2 Ohm Resister

Lake-Effect does the 6amps hold true for charging the battery thru the resister from the large battery.I looked at the small battery today and it's a 40ah battery.
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Old 20-06-2013, 15:05   #28
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Re: 2 ohm resister

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Originally Posted by tuberider View Post
I was looking on line at the xantrex echo-chargers. It says that it limits the amps to 15, so with the #12 wire going from the main battery to the 100ah battery wouldn't this echo-charger work. Is that what this device does ,limits the amps to no more the 15.
Yes. However, the EchoCharge is an all solid-state voltage follower device which is meant to maintain the START battery by bleeding a bit of current from the HOUSE batteries whenever they are under charge (i.e., over 13VDC). Start batteries don't require much charging...it takes less than 0.5AH to start the average diesel auxiliary.

I'd suggest you forget trying to figure out what the 2-ohm resistor does -- if it really is a resistor -- and ignore all the learned and not-so-learned discussion about the propriety of such usage on a boat. Just install an EchoCharge and forget about it.

The EchoCharge will maintain your start battery automatically...no switching or monitoring required. The one on my boat has been working 24/7 for about 8 years, and the many I've installed on customer's boats are working fine. BTW, this is about the only Xantrex product I'd recommend.

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Old 20-06-2013, 17:05   #29
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Quote:
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OK, I stand corrected , it was resistor
IMO, it is a crazy arrangement.
Why , it makes perfect electrical sense

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Old 20-06-2013, 17:07   #30
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Suggesting a echo charge for this application is way of the mark. The resistor will do just fine


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